Patent GB-1037181-A (GB)
Title: Production of polyphosphates
IPC: C01B25/445
Filed: 1963-04-19
Abstract: Alkali metal polyphosphates are obtained by heating a mixture of an alkali metal salt of a volatile acid and a water-soluble material selected from ammonium phosphate salts and mixed ammonium-alkali metal phosphate salts at above 200 DEG  C., the overall ratio of alkali metal to phosphorus being between 0.75 and 3. The alkali metal salt is one which decomposes at below 600 DEG  C. in a concentrated phosphoric acid medium, and the metal may be sodium, potassium, lithium, rubidium or caesium.  The volatile acid is one which may be removed, or its degradation products removed, by distillation from a concentrated phosphoric acid medium at below 600 DEG  C. at atmospheric pressure, e.g. hydrofluoric, hydrochloric, hydrobromic, hydriodic, nitric, nitrous, carbonic, oxalic and formic acids.  The ammonium phosphate salts, of water solubility at least 5% at 25 DEG C., may be mono-, di- and triammonium orthophosphates, ammonium pyrophosphates, trimetaphosphates, tetrameta phosphates, tripolyphosphates, the ammonium salts of polymeric phosphoric acids with more than four phosphorus atoms per molecule, and mixtures thereof, and polyammonium polyphosphates formed by reaction of NH3, H2O and P2O5. The mixed ammonium-alkali metal phosphates may be monosodium monoammonium orthophosphate, disodium diammonium orthophosphate and monopotassium tetra-ammonium tripolyphosphate.  At least half the alkali metal requirement is preferably in the form of the volatile acid salt.  The ratio of ammonium ion to metal is at least 1, except when the metal carbonate, bicarbonate, formate or oxalate is utilized when it may be as low as 0.7.  Preferably alkaline earth metal cations are absent. Alkali metal trimetaphosphate is obtained when the ratio of alkali metal to phosphorus is 1, tripolyphosphate when the ratio is 1.67 and pyrophosphate when the ratio is 2.  With intermediate ratios mixtures are obtained. Ratios between 0.75 and 1 give "ultraphosphates," and between 2 and 3 mixtures of pyrophosphate and orthophosphate are obtained. When the ratio is above 2, the more alkaline reacting alkali metal salts are utilized, e.g. sodium carbonate, potassium carbonate, sodium oxalate and formate.  The mixture is prepared by blending crystals of the reactants in e.g. a ribbon mixer, or by mixing solutions and drying, or, less desirably, by mixing the molten reactants.  The mixture is heated in e.g. an externally heated stainless steel drum, to a temperature at least 50 DEG  C. higher than that at which it initially melts, but preferably to below the melting point of the polyphosphate being prepared, and maintained at this temperature for at least 5 minutes.  The reaction temperature is generally above 300 DEG  C.  Preferably the temperature of the mixture is raised over a period of at least 10 minutes through at least 150 DEG  C. after the first gaseous decomposition products are evolved, which usually occurs about 110 DEG  C.  The end of the reaction is indicated when the reaction mixture becomes solid, e.g. between 350 DEG  C. and 625 DEG  C. The product is cooled to ambient temperature. Alternatively the reaction mixture may be quenched while still molten.  In examples the specified alkali metal salts are sodium chloride, sodium carbonate, sodium nitrate, potassium chloride, potassium carbonate, potassium chlorate and mixtures thereof.